1. Field of the Invention
This invention relates to light emitting diodes and more particularly to new structures for enhancing their light extraction.
2. Description of the Related Art
Light emitting diodes (LEDs) are an important class of solid state devices that convert electric energy to light and commonly comprise an active layer of semiconductor material sandwiched between two oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. The light generated by the active region emits in all directions and light escapes the device through all exposed surfaces. Packaging of the LED is commonly used to direct the escaping light into a desired output emission profile.
As semiconductor materials have improved, the efficiency of semiconductor devices has also improved. New LEDs are being made from materials such as GaN, which provides efficient illumination in the ultra-violet to amber spectrum. Many of the new LEDs are more efficient at converting electrical energy to light compared to conventional lights and they can be more reliable. As LEDs improve, they are expected to replace conventional lights in many applications such as traffic signals, outdoor and indoor displays, automobile headlights and taillights, conventional indoor lighting, etc.
However, the efficiency of conventional LEDs is limited by their inability to emit all of the light that is generated by their active layer. When an LED is energized, light emitting from its active layer (in all directions) reaches the emitting surfaces at many different angles. Typical semiconductor materials have a high index of refraction (n≈2.2-3.8) compared to ambient air (n=1.0) or encapsulating epoxy (n≈1.5). According to Snell""s law, light traveling from a region having a high index of refraction to a region with a low index of refraction that is within a certain critical angle (relative to the surface normal direction) will cross to the lower index region. Light that reaches the surface beyond the critical angle will not cross but will experience total internal reflection (TIR). In the case of an LED, the TIR light can continue to be reflected within the LED until it is absorbed, or it can escape out surfaces other than the emission surface. Because of this phenomenon, much of the light generated by conventional LEDs does not emit, degrading efficiency.
One method of reducing the percentage of TIR light is to create light scattering centers in the form of random texturing on the surface. [Shnitzer, et al., xe2x80x9c30% External Quantum Efficiency From Surface Textured, Thin Film Light Emitting Diodesxe2x80x9d, Applied Physics Letters 63, Page 2174-2176 (1993)]. The random texturing is patterned into the surface by using sub micron diameter polystyrene spheres on the LED surface as a mask during reactive ion etching. The textured surface has features on the order of the wavelength of light that refract and reflect light in a manner not predicted by Snell""s Law due to random interference effects. This approach has been shown to improve emission efficiency by 9-30%.
One disadvantage of surface texturing is that it can prevent effective current spreading in LEDs which have poor electrical conductivity for the textured electrode layer, such as the case of p-type GaN. In smaller devices or devices with good electrical conductivity, current from the p and n-type layer contacts spreads throughout the respective layers. With larger devices or devices made from materials having poor electrical conductivity, the current cannot spread from the contacts throughout the layer. As a result, part of the active layer does not experience the current and will not emit light. To create uniform current injection across the diode area, a spreading layer of conductive material is deposited on its surface. However, this spreading layer often needs to be optically transparent so that light can transmit through the layer. When a random surface structure is introduced on the LED surface, an effectively thin and optically transparent current spreader cannot easily be deposited.
Another method of increasing light extraction from an LED is to include a periodic patterning in the emitting surface or internal interfaces which redirects the light from its internally trapped angle to defined modes determined by the shape and period of the surface. See U.S. Pat. No. 5,779,924 to Krames et at. This technique is a special case of a randomly textured surface in which the interference effect is no longer random and the surface couples light into particular modes or directions. One disadvantage of this approach is that the structure can be difficult to manufacture because the shape and pattern of the surface must be uniform and very small, on the order of a single wavelength of the LED""s light. The pattern can also present difficulties in depositing an optically transparent current spreading layer as described above.
An increase in light extraction has also been realized by shaping the LED""s emitting surface into a hemisphere with an emitting layer at the center. While this structure increases the amount of emitted light, its fabrication is difficult. U.S. Pat. No. 3,954,534 to Scifres and Burnham discloses a method of forming an array of LEDs with a respective hemisphere above each of the LEDs. The hemispheres are formed in a substrate and a diode array grown over them. The diode and lens structure is then etched away from the substrate. One disadvantage of this method is that it is limited to formation of the structures at the substrate interface, and the lift off of the structure from the substrate results in increased manufacturing costs. Also, each hemisphere has an emitting layer directly above it, which requires precise manufacturing.
U.S. Pat. No. 5,793,062 discloses a structure for enhancing light extraction from an LED by including optically non-absorbing layers to redirect light away from absorbing regions such as contacts and also redirect light toward the LED""s surface. One disadvantage of this structure is that the non-absorbing layers require the formation of undercut strait angle layers, which can be difficult to manufacture in many material systems.
Another way to enhance light extraction is to couple photons into surface plasmon modes within a thin film metallic layer on the LED""s emitting surface, which are emitted back into radiated modes. [Knock et al., xe2x80x98Strongly Directional Emission from AlGaAs/GaAs Light Emitting Diodesxe2x80x99 Applied Physics Letter 57, pgs. 2327-2329 (1990)]. These structures rely on the coupling of photons emitted from the semiconductor into surface plasmons in the metallic layer, which are further coupled into photons that are finally extracted. One disadvantage of this device is that it is difficult to manufacture because the periodic structure is a one-dimensional ruled grating with shallow groove depths ( less than 0.1 xcexcm). Also, the overall external quantum efficiencies are low (1.4-1.5%), likely due to inefficiencies of photon to surface plasmon and surface plasmon-to-ambient photon conversion mechanisms. This structure also presents the same difficulties with a current spreading layer, as described above.
Light extraction can also be improved by angling the LED chip""s side surfaces to create an inverted truncated pyramid. The angled surfaces provide TIR light trapped in the substrate material with an emitting surface within the critical angle [Krames et al., xe2x80x98High Power Truncated Inverted Pyramid (AlxGa1xe2x88x92x)0.5In0.5P/GaP Light Emitting Diodes Exhibiting  greater than 50% External Quantum Efficiencyxe2x80x99 Applied Physics Letters 75 pgs. 2365-2367 (1999)]. Using this approach external quantum efficiency has been shown to increase from 35% to 50% for the InGaAlP material system. This approach works for devices in which a significant amount of light is trapped in the substrate. For the case of GaN on sapphire, much of the light is trapped in the GaN film so that angling the LED chip""s side surfaces will not provide the desired enhancement.
Still another approach for enhancing light extraction is photon recycling [Shnitzer, et al., xe2x80x98Ultrahigh spontaneous emission quantum efficiency, 99.7% internally and 72% externally, from AlGaAs/GaAs/AlGaAs double heterostructuresxe2x80x99, Applied Physics Letters 62, Page 131-133 (1993)]. This method relies on LEDs having a high efficiency active layer that readily converts electrons and holes to light and vice versa. TIR light reflects off the LED""s surface and strikes the active layer, where the light is converted back to an electron-hole pair. Because of the high efficiency of the active layer, the electron-hole pair almost immediately reconverts to light that is again emitted in random direction. A percentage of this recycled light strikes one of the LEDs emitting surfaces within the critical angle and escapes. Light that is reflected back to the active layer goes through the same process again. However, this approach can only be used in LEDs made from materials that have extremely low optical loss and cannot be used in LEDs having an absorbing current spreading layer on the surface.
The present invention provides a class of new LEDs having interconnected arrays of micro-LEDs to provide improved light extraction. Micro-LEDs have a smaller active area, in the range of 1 to 2500 square microns, but the size is not critical to the invention. An array of micro-LEDs is any distribution of electrically interconnected micro-LEDs. The arrays provide a large surface area for light to escape each of the micro-LEDs, thereby increasing the usable light from the LED. The new LED can have many different geometries and because it is formed by standard semiconductor process techniques, it is highly manufacturable.
The new LED includes a conductive first spreader layer with micro-LEDs disposed on one of its surfaces. Each micro-LED has a p-type layer, an n-type layer and an active layer sandwiched between the p- and n-type layers. Either the p- or n-type layer is a top layer and the other is the bottom layer. Current applied to the first spreader layer spreads into each micro-LED""s bottom layer. A second spreader layer is included over the micro-LEDs and current from said second spreader spread into the top layer. When a bias is applied across the first and second spreader layers the micro-LEDs emit light.
One embodiment of the second spreader is a conductive interconnected grid-like structure having conductive paths over the micro-LEDs, in contact with the top layer of the micro-LEDs. An insulator layer is included over the array with the grid on the insulator layer, thereby electrically isolating the first spreader layer from the grid.
Alternatively, flip-chip bonding can be used to interconnect the micro-LEDs. Using this method, an unconnected micro-LED array is first formed and then bonded to an electrically conductive material to provide the array interconnection. In a third embodiment, the grid passes over the micro-LEDs and the p-type, active, and n-type material is under the conductive paths of the grid between the micro-LEDs to electrically isolate the grid from the first spreader layer. This grid-like structure can be designed so that emitted light interacts with a sidewall after traveling a small distance.
The new LED can have LEEs disposed between the micro-LEDs or formed on the side surfaces of the micro-LEDs, to further enhance light extraction. The LEEs act to redirect or focus light that would otherwise be trapped or absorbed through TIR in a standard LED structure. Their shapes may be curved (convex or concave) or piecewise linear with the shape of the structure affecting the light extraction and final output direction of light. LEEs that are placed between the micro-LEDs interact with light escaping from the sides of the micro-LEDs. This interaction helps prevent the light from reflecting back into the LED to be absorbed, thereby increasing the useful light out of the LED.
These and other further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which: